What caused a radio blackout in cities across US?

A "strong radio blackout" was reported in several parts of the US by the National Oceanic and Atmospheric Administration's (NOAA) Space Weather Prediction Centre, soon after it was predicted that solar flares would disrupt communications signals on Earth resulting from increased activity on the sun's surface.

"The flare came from a large and complex region denoted as sunspot group 3354," NOAA said, adding that it led to a "temporary degradation or complete loss of High Frequency (HF) radio signals on some of the sunlit sides of Earth."

According to The Independent, space weather scientists are concerned about the giant sunspot, known as AR3354, which was first discovered on June 27 and rapidly grew in size over the course of two days to a size roughly 10 times larger than Earth.

Nasa stated: "Sunspots are areas that appear dark on the surface of the Sun. They appear dark because they are cooler than other parts of the Sun's surface.

"The magnetic field lines near sunspots cause explosions as they reorganise, causing solar flares. It releases a lot of radiation into space. The intense explosion, releases radiation that can interfere with our radio communications here on Earth."

In addition to this, solar storms pose a risk because they cause massive coronal mass ejections (CME) to be sent into space and strike the magnetic field of the Earth.

Geomagnetic storms may be triggered by their effects. 

As a result, it is possible that the satellites, communication, internet connectivity, and GPS will be interfered with or harmed. Failures of the power grid might also result from it.

Therefore, scientists predict that as the sun becomes more active, more sunspots, solar flares, and CMEs will develop.

The progression of the solar cycle towards maximum activity can be observed by scientists by tracking sunspots.

It was predicted by a panel of scientists in 2020 that the sun's current cycle would reach its maximum in 2025 with a peak of roughly 115 sunspots.

However, the number of sunspots outstripped the predictions, and in January 140 sunspots were seen when no more than 92 were predicted, according to a NOAA database. In May, there were 140 sunspots.

The solar storm Sunday led to brief radio blackouts lasting for about 30 minutes in the western US and parts of the eastern Pacific Ocean, according to NOAA.

The flare peaked in intensity at 7:14pm ET on July 2, Nasa’s Solar Dynamics Observatory said in a statement.

Experts noted that if the flare had launched, along with it, fast-moving charged particles, in what’s called a CME, it might have led to an even larger radio blackout.

The sunspot AR3354 has not contracted and may still emanate more solar flares in the days to come.

This sunspot first produced a minor M-class flare on June 29 and, after a brief period of calm, unleashed an X-class flare — the most powerful category of solar flare the Sun can belch out — aimed directly at Earth.

"When this week began, sunspot AR3354 didn’t exist. Now it is 10 times wider than Earth and still growing. The sunspot burst into view on June 27th, breaching the surface of the sun, then blossoming into a giant over the next 48 hours," noted Spaceweather.com.